The cam is a dot, so to say... A dot which shoots rays in all the directions, where you see something on your screen.

An other way to reduce DoF is to use a more zoomed lens, I think...
(So, but your camera further away but adjust the filmsize in order to get the same effect...)
- though, this causes things to look flat

It's a pain working at such small sizes - here's a tip: create your scenes at 10x real scale (so that it's easier to work with), then use the "Scene Scale" adjustment in Blendigo to reduce it on export! So much easier than dealing with blender and very small units

EDIT2: Real diamond has an IOR of 2.417ish - i'm looking up the cauchy or sellmier measurement now

EDIT3:
Couldn't find a cauchy number anywhere, but .04 as you had it looks good.
To fix your focus, I changed the focal length to 125mm and instead moved the camera slightly closer to compensate, then changed the film width to 22mm.

EDIT4: Well that improves the situation, but does not fix it. I'm going to just change the aperture size directly, because f/64 isn't high enough for this particular scene.

Changes from yours:
Scene scaled by 10x.
The modifiers you had on the ring caused holes in the mesh to appear (edgesplit then subsurf) - I approximated what you intended with bevel then subsurf then edgesplit.
Ground plane moved up to make ring "rest" on it.
Camera focal length changed to 125mm.
Camera moved closer to compensate.
IOR for diamond changed to 2.417
Scene scale setting in blendigo set to .1x

Upon export:
go into default.igs and add four zeroes after the decimal point for "aperture width"